1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copiers, printers, facsimiles and multi-functional peripherals thereof.
2. Description of the Related Art
The image forming apparatuses such as copiers, printers, facsimiles and multi-functional peripherals thereof are generally arranged such that a developing unit supplies a toner to an image carrier with an electrostatic latent image formed thereon, thereby forming a toner image on a surface of the image carrier in correspondence to the electrostatic latent image.
The above developing unit is known to include a developing unit of a mono-component development system which uses a toner alone as a developer and a developing unit of a two-component development system which uses a developer including the toner and a carrier. The developing unit of the two-component development system using the developer including the toner and the carrier is superior in terms of quick and adequate toner charging for facilitating high-speed image formation.
The above developing unit of the two-component development system normally takes the following steps to supply the toner for developing the electrostatic latent image formed on the image carrier. The developing unit mixes and agitates the developer including the toner and the carrier to thereby charge the toner present in the developer by contact with the carrier. The developer with the toner thus charged is retained on a developer carrier, which delivers the developer to a development position opposite the image carrier. The toner present in the developer retained on the developer carrier is supplied to the image carrier whereby the toner image corresponding to the electrostatic latent image is formed on the surface of the image carrier.
In a case where the developer is decreased in toner content in consequence of the above-described toner supply to the image carrier, a toner replenishing mechanism replenishes the developing unit with the toner.
After the toner image corresponding to the electrostatic latent image is formed on the image carrier by supplying the toner from the developing unit, a transfer unit operates to transfer the toner image formed on the image carrier to a transfer receiving body such as a recording medium or an intermediate transfer belt. After the transfer of the toner image, the toner remaining on the surface of the image carrier is recovered by a toner recovery unit.
More recently, there has been proposed an image forming apparatus adapted for effective use of the toner thus recovered by the toner recovery unit. In this apparatus, the toner recovered by the toner recovery unit is transported to a return transport mechanism by means of a recovered toner transporting member disposed in the toner recovery unit. The return transport mechanism returns the recovered toner to the developing unit.
Japanese Patent No. 3352399, for example, proposes an apparatus wherein the toner recovered by the toner recovery unit is transported by transport means to a recovered toner reservoir so as to be stored therein and wherein the recovered toner from the recovered toner reservoir and fresh toner from a fresh toner reservoir are blended in a proper ratio and the resultant toner mixture is supplied to the developing unit to be used for image development. The Japanese Patent No. 3352399 also proposes an arrangement to deal with a case where the toner collected in the recovered toner reservoir exceeds a predetermined volume. In such a case, a transport path for transporting the recovered toner is switched from the recovered toner reservoir to the fresh toner reservoir containing the fresh toner so that the toner collected in the recovered toner reservoir may not become agglomerated as subjected to pressure increase or the like.
The above-described developing unit of the two-component development system also has a problem that while the developer is used over an extended period of time, the carrier in the developer is gradually deteriorated to become incapable of adequately charging the toner. As a result, the developing unit fails to accomplish proper image formation.
More recently, therefore, there is a practice in the art to remove the above-described developer suffering the carrier deterioration from the developing unit for replacement with a fresh developer or to replace the developing unit per se with a new one.
However, in the case where the developer suffering the carrier deterioration is removed from the developing unit for replacement with the fresh developer or where the developing unit per se is replaced with the new one, the developer to be replaced actually contains a substantial amount of toner that is still usable. Namely, there is a problem that the usable toner remaining in the pre-replacement developer is wastefully discarded.
In this connection, Japanese Patent No. 3581720 discloses an apparatus wherein the above developing unit is provided with a developer recovery portion and a toner recovery portion as separation means for separating the toner from the developer being recovered. Specifically, a charge of the opposite polarity to that of the toner is applied to a developer recovery sleeve so as to separate the toner from the developer being recovered. The separated toner is recycled back into the operation.
However, the following problems exist with the apparatus wherein the developing unit is provided with the developer recovery portion and the toner recovery portion as the separation means for separating the toner from the developer being recovered and wherein the toner is separated from the developer being recovered by applying to the developer recovery sleeve the charge of the opposite polarity to that of the toner. That is, the developing unit requires much equipment which leads to cost increase. What is more, the developing unit is increased in size.